In many cases where dissolved or suspended solids must be separated from a liquid in such a way as to recover a dry solid, a common approach is to use spray drying in which the liquid containing the solids is sprayed into a hot gas. In this process the gas is cooled; the liquids are vaporized, and the solids are dried. This technique has the problem that the solids so produced must be separated from the gaseous components, thereby requiring further processing. In one approach cyclones are used to separate the solids. Other approaches use filter elements such as cloth bags, electrostatic precipitators, and filter elements of sintered metal or ceramic or mineral wool fibers.
The present invention is concerned with the situation where the liquid phase which is evaporated contains a high boiling point corrosive mineral acid such as sulphuric acid and the gases which are produced must be kept at elevated temperatures to avoid condensation and corrosion. For such an application classic filtration materials such as cloth bags cannot be used and gas cleaning must take different approaches. The equipment must also be kept hot as the dewpoints of such gases can be in the range 200.degree. to 300.degree. C.
It will be realised that for spray drying using hot gases, there must be sufficient heat in the gases that all of the liquids can be evaporated and the solids heated all to a temperature above the dewpoint of the mixture. Drying however is not an instantaneous phenomenon and residence times of 20 to 60 seconds in the spray dryer are typical. Because of the relatively long residence time and the large volumes usually involved, spray dryer vessels are typically very large field fabricated units.
If electrostatic precipitators are used for cleaning, gas residence times are typically under 10 seconds which is only a fraction of the time required for drying. If cyclones are used, even less residence time is involved. If cleaning is performed by filtration, the gas will pass through a filter cloth or medium such as sintered metal in a small fraction of a second and the residence time associated with the filtration operation will be essentially the time required to distribute gas to the filter surface and the collection of the gas after filtration. One evaluation concerning the spray drying of a smelter weak acid resulted in a spray dryer vessel with an internal volume of 6,700 cubic feet associated with a sintered metal filter containing 357 filter candles in a vessel with a filter element volume of 186 cubic feet. For this case it was proposed to house the 357 associated candles in a separate vessel 11 feet in diameter by 16 feet high, as compared with the spray dryer vessel which was 20 feet in diameter and 35 feet in overall height. This application was not a large unit and for larger flows, it would be normal practice to use multiple filter vessels with manifolds to distribute to and collect the gases from each filter vessel.
While the above approach is technically feasible, it poses a number of problems for the owner and operator of a plant and better solutions would be desirable. Among the problems with the above approach are the following.
Gases containing sulphuric acid and water in the form of vapor are well known as requiring to be kept hot to avoid corrosion, since most materials are corroded by hot sulphuric acid condensate. Such protection is normally achieved by keeping the gas temperature well above the dewpoint and rigorously insulating the exposed surfaces to prevent heat losses. Such protection also requires that structural steel and all materials which contact the surface be prevented from cooling, and therefore complicates plant and equipment design. The costs and complexities introduced make the simplification of the design a feature of significant value.
A second consideration in keeping a vessel hot is that heat is always being lost and will be lost in proportion to the surface exposed. Any design approach should therefore minimize exposed hot surface.
A further problem is one of scale. The example given above for a spray dryer vessel with sintered metal filter candles represented the largest single filter vessel presently commercially available, and larger flows therefore require multiple units. Multiple units require gas ducting and additional surfaces which require insulation and protection against heat loss. This is costly in terms of capital and operating costs and consumes unnecessary energy.
Classic spray dryer design involves gas and liquids entering in the upper portion of a spray dryer vessel and flowing downward, leaving from the side or bottom. While droplets clearly will fall, taking gas off from a lower portion of the vessel is likely to entrain the solids which one wishes to separate from the gas, and makes the gas cleaning job more difficult. Any approach which takes the gases off from a point where the solids are less likely to be present is therefore likely to be beneficial to the downstream operation.
In addition, many of the solids to be handled in spray drying operations will contain toxic substances or elements such as heavy metals, arsenic, etc and must therefore be closely contained for the safety of plant personnel and the outside environment. Often the spray vessel may be under pressure and leakage around solid takeoffs is difficult to control. Any design approach which generates more than one such takeoff point therefore may create additional hazards for the plant operator and the environment.
Further disadvantages with complex systems include the tendency of solids to settle out in ducts and piping with unknown consequences to the equipment, a lessened ability to ensure economically that all equipment is properly protected against corrosion (e.g. by keeping it above the dewpoint temperature), and the inherent extra expense of maintaining such complex systems.